The forum folks at DigitalPoint are discussing what is the best keyword density for your AdSense ads to show the most relevant ads. Some member say you want at least a total "around 200 words with a 4% density" to get the most relevant ads. And here is a quote from page two.
go for 4% but not from the get go. I kind of work my way up there. The problem is, it could take anywhere from 2 days to 2 weeks for the change to take effect.
To complicate matters a change may take effect very quickly within a few hours only to return to the original in a day or two! So you need to monitor it carefully.
Also, in my opinion the mediapartners bot prefers to “see” a couple of keyphrases so I always target say “striped blue widgets” 2.5%, “blue widgets” by itself around 3.5% and 1% or so of “blue striped widgets” while all the time target is really “blue widgets”.
Note that the above densities are not cumulative. Total “blue widgets” density never exceeds 4%
Talk about watching AdSense like a hawk. :)